The present invention relates to a working distance detecting device in an ophthalmic apparatus.
In an ophthalmic apparatus such as a funduscopic camera, illuminating beams are projected on an eye to be examined through an object lens confronting to the eye to be examined, and in case of an eye fundus camera, an arrangement is made such that reflected beams of the projected light from the fundus are guided to a photographic optical system. In such eye fundus cameras as well as other ophthalmic apparatus in which illuminating beams are projected on an eye to be examined through an object lens and a flux of beams reflected from the interior of the eye to be examined is guided to a photographic optical system or an observation optical system, there is a risk of occurrence of a flare or ghost because of intrusion of illuminating beams reflected from the face of the cornea of the eye being examined, into the photographic or observation optical system.
As means for overcoming this problem of occurrence of a flare or ghost by a flux of reflected beams of illuminating light, conventional apparatus include an illuminating optical system which has an annular or ring-shaped aperture and an apertured mirror slantingly disposed along the optical axis of the object lens at a position substantially conjugate to the pupil of an eye to be examined with respect to the object lens so that a ring-shaped flux of illuminating beams passing through the ring-shaped aperture is once focussed on an annular reflection surface of the apertured mirror and the flux is then made incident into the eyeball through the pupil of the eye being examined from the object lens. If this arrangement is adopted, occurrence of a flare or ghost due to reflection of illuminating beams on the cornea can be prevented when the perforated mirror is located at a position substantially conjugate to the pupil of the eye to be examined with respect to the object lens. More specifically, since the distance between the object lens and the apertured mirror is always constant, when the distance between the object lens and the eye to be examined, that is, the working distance, is appropriate, occurrence of a flare or ghost can be prevented.
Accordingly, in an ophthalmic apparatus of this type, it is very important to maintain an appropriate working distance while the apparatus is used. This appropriate working distance has heretofore been obtained by precisely moving the apparatus by an operator while observing a picture image produced in an observation optical system, that is, a picture image produced on a Braun tube of a monitor television when illuminating beams are infrared rays or a picture image seen through an eye lens when illuminating beams are visible rays. When this conventional method is adopted, since not only this operation of adjusting the working distance but also other complicated operations such as adjustment of alignment of the optical axis of the object lens with the eye to be examined and the focusing should simultaneously be performed, a certain degree of flare or ghost is apt to be overlooked. Furthermore, in an ophthalmic apparatus of the type where the operation of alignment adjustment and the focussing operation are carried out by using infrared rays, it is very difficult to detect rays diffused on the cornea or the crystalline lens on a Braun tube of a television.
As means for eliminating this disadvantage, a proposal has been made by Japanese Patent Application No. 81631/79. According to this proposal, a light-receiving portion is disposed in the peripheral region of the object lens and the working distance is detected in terms of the position of the flux of beams reflected from the cornea on this light-receiving portion. The light-receiving portion may be constructed, for example, by a light-receiving plate arranged coaxially with the object lens in the peripheral region of the object lens. In this case, when the radius of the inner circumference of an image of illuminating beams reflected from the cornea, which is formed on the surface of the light-receiving plate, has a predetermined value, it is judged that the working distance is appropriate. Furthermore, the state of alignment of the object lens with an eye to be examined can be known by examining whether or not the inner circumference of the image formed on the surface of the light-receiving plate is coaxial with the optical axis of the object lens.
This working distance detecting device is very advantageous over the conventional ones because the working distance and the alignment can be detected very conveniently. Furthermore, if an optical fiber or photoelectric element is used instead of the light-receiving plate, any necessary display can easily be accomplished on an image forming portion of the observation optical system. However, in this device, since both the working distance and the alignment are detected only by detecting the position of the inner circumference of the image of the ring-shaped aperture formed by beams reflected at the cornea of the eye, actual adjustment involves some problem or difficulties.